AMBOCCELIA 423 



Genus AMBOCCELIA Hall 



Description. Shells small, plano-convex or slightly concavo-convex, 

 subcircular tG subsemielliptical in outline, the hinge-line usually a little 

 shorter than the greatest width of the shell, the cardinal extremities 

 rounded. Surface of valves smooth or with fine concentric lines of 

 growth, rarely minutely spinose, shell substance not punctate. Pedicle 

 valve strongly convex with very prominent umbonal region, the beak 

 strongly incurved, marked by a narrow, median, groove-like sinus which 

 is usually strongest posteriorly; cardinal area well defined, concave, the 

 delthyrium rather large and open, with incomplete deltidial plates. 

 Internally the hinge-teeth are prominent, with recurved tips, not sup- 

 ported by dental lamellae; the muscular area small, the interior surface 

 about the muscular scars strongly pitted. Brachial valve convex at the 

 beak, becoming flat or concave anteriorly, with a rather broad cardinal 

 area lying in nearly a right angle to the area of the opposite valve ; car- 

 dinal process bifurcate posteriorly, elongate, resting on the bottom of 

 the valve except at the posterior extremity, the spires of the brachidium 

 loosely coiled with few volutions, attached by long crura which are 

 continuous with the primary lamella, the jugum in the same rudimentary 

 condition as in Spirifer. 



Remarks. This genus of small shells may be easily recognized, in most 

 cases, from the detached pedicle valves, the condition in which the species 

 most commonly occur in our faunas, by reason of the smooth surface, the 

 prominence of the umbonal region and the narrow, groove-like mesial 

 sinus. The genus is represented by several species, all of which are 

 restricted to the Kinderhook faunas. 



AMBOCCELIA MINUTA White 



Plate LXXVII, Figs. 44-54 



1862. Amboccelia (Spirifer?) minuta White, Proc. Boston Soc. Nat. Hist., 

 vol. 9, p. 26. 



1894. Amboccelia minuta Keyes, Mo. Geol. Surv., vol. 5, p. 90. 

 1908. Ambocoelia minuta Rowley, Mo. Bureau Geol. and Mines, vol. 8, 

 2nd ser., p. 85, pi. 18, figs. 21-24. 



Description. Shell minute, subplanoconvex, subsemielliptical in out- 

 line, the hinge-line a little shorter than the greatest width of the shell, 

 the cardinal extremities rounded. The dimensions of an average in- 

 dividual are : length of pedicle valve 2.3 mm., length of brachial valve 

 2 mm., greatest width 2.2 mm., length of hinge-line 1.5 mm., thickness 

 1.9 mm., the largest examples observed do not exceed 3 mm. in maximum 

 length. 



Pedicle valve gibbous, the greatest depth opposite the hinge-line, the 

 umbo prominent, the surface curving abruptly from the umbonal region 



